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       pnmrotate - rotate a portable anymap by some angle


       pnmrotate [-noantialias] angle [pnmfile]


       Reads  a  portable  anymap as input.  Rotates it by the specified angle
       and produces a portable anymap as output.  If  the  input  file  is  in
       color,  the  output  will  be too, otherwise it will be grayscale.  The
       angle is in degrees (floating point), measured  counter-clockwise.   It
       can  be  negative,  but  it  should  be  between -90 and 90.  Also, for
       rotations greater than 45 degrees you may get  better  results  if  you
       first  use  pnmflip  to do a 90 degree rotation and then pnmrotate less
       than 45 degrees back the other direction

       The rotation algorithm is Alan Paeth’s three-shear method.  Each  shear
       is  implemented  by  looping  over  the  source pixels and distributing
       fractions to each of  the  destination  pixels.   This  has  an  "anti-
       aliasing"  effect  -  it  avoids  jagged  edges  and similar artifacts.
       However, it also means that the original colors or gray levels  in  the
       image  are  modified.   If  you  need to keep precisely the same set of
       colors, you can use the -noantialias flag.  This does the  shearing  by
       moving pixels without changing their values.  If you want anti-aliasing
       and don’t care about the precise  colors,  but  still  need  a  limited
       *number* of colors, you can run the result through ppmquant.

       All flags can be abbreviated to their shortest unique prefix.


       "A  Fast Algorithm for General Raster Rotation" by Alan Paeth, Graphics
       Interface ’86, pp. 77-81.


       pnmshear(1), pnmflip(1), pnm(5), ppmquant(1)


       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                12 January 1991                   pnmrotate(1)